Containers intended to contain a still liquid (for example bottles intended to contain drink water) are, in the majority of cases, provided with a rounded base in the general form of a spherical cap having a concavity turned outwards and of relatively small height. Such bases are often provided with substantially radially radiating ribs which are distributed around a central recess, said ribs possibly having various shapes and optionally extending possibly onto the lower part of the wall of the body in order to reinforce the foundation (peripheral zone with which the base rests on a support). The height of bases of this type, including the central reinforcement, is typically of the order of 10 mm, and can be up to 15 mm.
Such bases are suitable for withstanding, without deformation, the column of still liquid which rises above them. However, they do not offer sufficient resistance to withstand an additional stress, even though small, that may be due for example to an internal excess pressure.
Now, it is known, during the packaging of certain readily oxidizable still liquids (for example oil, fruit juices), to pour a small liquid quantity (for example one drop) of an inert substance that evaporates quickly (for example generally nitrogen) onto the surface of the still liquid at the end of the phase of filling the container in order to remove the air (and therefore the oxygen contained therein) from the free volume rising above the liquid surface immediately before the sealing of the container (an operation known as “inertization” or “nitrogenation”) or to improve the pressurization of the container in the case of weakly carbonated liquids. This small quantity of inert substance ceases to evaporate once the sealing has finished, such that inertization gas remains in the sealed container under a small residual pressure below 2×105 Pa, typically of the order of 1×105 Pa, even of the order of 0.5×105 Pa.
The weakly rounded bases traditionally provided for containers intended for still liquids cannot reliably withstand, without deformation, even a pressure as small as that generated by the inertization process.
It is also known, for containers whose contents must undergo an inertization process, to provide them with bases that are improved in terms of resistance such that they do not deform under the action of the internal excess pressure.
A reinforced base of this type which is normally used at the present time is illustrated in FIGS. 1A to 1D of the attached drawings. FIG. 1A is a side view of the lower part of a container 1′ (here a bottle having the general shape of a substantially cylindrical body of revolution) made of a thermoplastic material such as PET, having a body 2′ extending between, at the top, a neck (not shown) and, at the bottom, a base 3′. The base 3′ is represented, alone, in FIG. 1B in a bottom view, in FIG. 1C in diametral section along the line IC-IC of FIG. 1B, and in FIG. 1D in perspective from below.
The base 3′ comprises an arch 4′ having a rounded general shape with a concavity turned towards the outside of the container 1′ and it has an annular zone 5′ surrounding the arch 4′ and forming a substantially flat foundation with which said base 3′ can stably rest on a flat support. In the central part thereof, the arch 4′ opens onto a rounded dome 6′ which also has a concavity turned outwards, said dome being therefore situated offset towards the inside of the container relative to the arch. Outside the annular zone 5′ forming a foundation, the base 3′ has a wall 8′, turned inward, also called connecting wall 8′ for connection with the wall 9′ of the body 2′ of the container.
Several main ribs 7′ opening outwards, having the general shape of troughs with substantially parallel edges and having a substantially constant depth, extend radially in a star pattern from the inward-turned wall 8′ of the base 3′ to the dome 6′ in which they end, while crossing the annular zone 5′ forming a foundation and the arch 4′; in the example illustrated, there is an odd number of main ribs 7′, here equal to five. To improve the mechanical resistance of the base, secondary ribs 10′ are added, which can have substantially the same structure as the main ribs 7′, which are interspersed between the main ribs 7′, but extend radially in a star pattern from the inward-turned wall 8′ of the base 3′ only as far as the middle of the arch 4′, while crossing the annular zone 5′ forming a foundation.
It will be emphasized that all the ribs, both the main ribs 7′ and the secondary ribs 10′, are formed sunken in the arch 4′, which has a smooth annular shape notched only by the ribs, as is clear from FIGS. 1A to 1D.
Manufacturers of containers made of a thermoplastic material such as PET constantly seek to make the containers lighter, which is reflected in, among other things, a lightening of the bases of the containers. For this reason, bases of containers having shapes which were satisfactory a few years ago are no longer suitable, because of the perceptible reduction in the quantity of material used.
Thus experience showed that a reinforced base designed as described above was no longer satisfactory, in a lightened version thereof, even for excess pressures of only approximately 1×105 Pa.
Now, the distribution of the drops of inertization liquid requires, in order to be precise, dosing equipment which is relatively costly and which manufacturers avoid using. In these conditions, the drops of inertization liquid are formed in a more or less empirical manner and the volume thereof can vary very substantially, in practice between half and double the desirable theoretical value. As a result, the excess pressure generated inside the container can be much higher than the desired value of approximately 1×105 Pa and can reach up to 2×105 Pa. The aforementioned bases, in their lightened version, cannot thus reliably withstand such an excess pressure without yielding.
From document FR 2 883 550, a container provided with a reinforced base which was intended for the same application is admittedly known. However, this also relates to a container intended to be manufactured with a substantial quantity of thermoplastic material and which does not make it possible to obtain the lightened version that manufacturers require today.
It is admittedly known, in the case of carbonated liquids (for example with a pressure of about 3 to 4×105 Pa, even up to 10×105 Pa), to design containers, the base of which has a much more pronounced curvature (called “champagne base” or similar) adapted for withstanding relatively high pressures without deformation. However, such bases require an increased quantity of thermoplastic material, both because of their greater height and because of the increased wall thickness, at least locally, in the annular zone forming a foundation. Containers provided with such bases therefore prove to be more costly and their correct shaping during moulding is more tricky. It is therefore not desirable to provide bases of this type to containers subjected to an inertization step, even less so as they are shaped to withstand much higher pressures than those created by the inertization operation: their high resistance and the extra cost which is associated with them appear excessive for the envisaged application.